Plasma processing within an evacuable processing chamber is commonly used in semiconductor device fabrication, such as for etching or depositing materials on a semiconductor substrate. Radio frequency (RF) power is commonly used to generate the plasma within the processing chamber. It is important, therefore, to accurately control the RF power delivered to the plasma within the chamber. Accurate control of plasma processing conditions is essential to achieve the relatively small critical dimensions and precise tolerances required in fabricating modern integrated circuits. Accurate control of the power delivered to the plasma is also important, for example, to detect the end point of an etching operation.
In a typical RF plasma processing apparatus 10 of the prior art as shown schematically in FIG. 1, an RF generator 11 is coupled to a shielded plasma generating electrode 12 positioned within an evacuable processing chamber 13. The connection to the electrode 12 made via an insulated feedthrough 14 extending through a wall of the processing chamber 13. The RF source 11 is coupled to the plasma generating electrode 12 through a conventional power meter 15, matching network 16, and RF connection 17. The processing chamber 13 may be evacuated using an exhaust port 18. The RF power delivered by the electrode 12 interacts with a process gas introduced into the chamber 13 through an inlet, not shown, to thereby produce the plasma 20 for semiconductor processing.
Accurate control of the power delivered to the plasma requires that the power delivered to the plasma be first readily and accurately determined or measured. As complete layers of a material are etched away by the plasma, for example, it is important to sense the resultant change in the impedance of the plasma and alter the processing conditions responsive to the sensing of these end point conditions. Unfortunately, conventional devices and techniques for determining the power delivered to the plasma generating electrode are not sufficiently accurate for many such applications.
As shown in FIG. 1, in a typical conventional RF plasma processing application, a torroidal current sensor 21 and high voltage sensor 22 have been used to determine the power delivered to the plasma generating electrode 12, and hence the power delivered to the plasma 20, from the RF generator 11. The torroidal current sensor 21 is coupled to the RF connection 17 from the RF generator 11. The outputs from the voltage sensor 22 and the torroidal current sensor 21 may be displayed on an oscilloscope or used by a microprocessor-based controller, not shown, to regulate the power delivered to the plasma 20 by the RF generator 11. A conventional RF plasma processing monitoring and control system of this type is commercially available from Comdel, Inc. of Beverly, Mass., under the model designation RPM-1.
Unfortunately, this type of indirect measurement of current using only a torroidal current sensor is susceptible to parasitic capacitance induced errors. More particularly, the errors result, in part, due to the placement of shielding relative to the current carrying inner conductor and the position of the shield relative to ground in the matching network where the torroidal sensor is typically installed.
Attempts have been made to determine the power delivered to the plasma in plasma processing apparatus using other approaches. For example, Japanese Patent No. 58-171822 to Ogawa entitled Dry Etching, discloses a probe inserted directly into plasma. The current flowing into the probe is measured by an ammeter through a high frequency filter. Another plasma-intrusive type of measuring device is disclosed in Japanese Patent No. 2-35724 to Fujita entitled Plasma Etching Monitoring Method. The Fujita patent discloses a Rogowski coil arranged between an upper electrode and a lower electrode and monitored by an integrator and oscilloscope to thereby determine the stability of etching and the end point of a particular etch. Similarly, Japanese Patent No. 63-280421 to Miyahara entitled Method and Apparatus for Controlling Working Depth of Dry Etching discloses a circuit for measuring the ion current flow using a probe in the ion current stream within the processing chamber.